A non-self-emitting display device, such as a liquid crystal display device, for example, includes an lighting device called a backlight or frontlight. The backlight is provided on the rear side of a liquid crystal display panel. Light which is emitted from the backlight and transmitted through the liquid crystal display panel is used for display. The frontlight is provided on the viewer side (front side) of the liquid crystal display panel. Light which is transmitted through a liquid crystal layer of the liquid crystal display panel and then reflected toward the viewer side is used for display.
An example of these lighting devices is an edge light type lighting device. The edge light type lighting device includes a light source and a light guide plate (a light guiding element in the form of a plate). In recent years, particularly, lighting devices in which a LED is used as the light source have been widely used. One or a plurality of LEDs are provided at one lateral surface or corner of a transparent light guiding element. Light emitted from the LEDs enters the light guiding element at the one lateral surface and propagates inside the light guiding element, before outgoing from the light guiding element. The ratio between the light propagating inside the light guiding element and the light outgoing from the light guiding element is adjusted according to, for example, the shape and density of recessed or raised portions formed in the surface of the light guiding element. For example, a minute prism pattern is formed across the viewer side surface of the light guiding element of the frontlight, so that light which is totally reflected at the interface between the light guiding element and the air propagates inside the light guiding element, and light which does not meet the total reflection conditions outgoes to the viewer side. By adjusting the angle of the slope surface of the minute prism and the length of the slope surface, the ratio between the propagating light and the outgoing light is determined.
Patent Documents 1 and 2 disclose light guiding elements which do not have a minute prism pattern in the surface. These light guiding elements have a plurality of light guiding layers (light guiding element pieces) which have different refractive indices, and the plurality of light guiding layers are arranged so as to form interfaces which are inclined with respect to the principal surface of the light guiding element. Part of light incoming from the lateral surface of the light guiding element is refracted at the interfaces so that the traveling direction of the light is changed, and for example, the light outgoes from the principal surface on the display panel side. Another part of the light incoming from the lateral surface of the light guiding element is reflected a plurality of times inside the light guiding element and by a reflection plate, and then refracted at the interfaces, before outgoing from the principal surface on the display panel side.
On the other hand, the applicant of the present application developed an antireflection film which has a moth-eye structure (sometimes referred to as “moth-eye type antireflection film”). The moth-eye structure has minute raised portions (which have a conical or bell-like shape), and the effective refractive index continuously varies. Therefore, the reflectance can be decreased to a level which is less than 1% and, furthermore, to a level which is not more than 0.2%. Further, the moth-eye type antireflection film is capable of preventing reflection over a wide wavelength range of light and has a wide incidence angle range as compared with an antireflection film which is manufactured with the use of a dielectric multilayer film (Patent Documents 3 to 6). One of the moth-eye structure fabrication methods which uses an anodized porous alumina layer that is obtained by anodization of aluminum is excellent in mass-productivity (Patent Documents 4 to 6). It is preferred that, in a moth-eye structure which is suitable for an antireflection film, the two-dimensional size of the raised portions (the diameter of the raised portions which have a conical or bell-like shape) is not less than 10 nm and not more than 500 nm, the distance between adjacent raised portions is also not less than 10 nm and not more than 500 nm, the height of the raised portions is not less than 100 nm and not more than 500 nm, and the two-dimensional distribution of the raised portions has no regularity. The entire disclosures of Patent Documents 3 to 6 are incorporated by reference in this specification.
Patent Document 7 discloses a light guiding element for front light, which has a moth-eye structure over its surface.